Error: MongoError: "Connection failed: Failed to decode base64"

After upgrading to mongo@v0.33.0, the connection consistently fails with the error "Connection failed: Failed to decode base64", occurring both locally and on Deno Deploy. I had to downgrade to mongo@v0.32.0 for now.

```ts
import { Database, MongoClient } from "mongo";

const DATABASE_URL = "mongodb+srv://:@/?authMechanism=SCRAM-SHA-1"

const client = new MongoClient();
await client.connect(DATABASE_URL);
```

Start by reproducing the failure with the `MongoClient.connect(DATABASE_URL)` example on mongo@v0.33.0, then compare it with v0.32.0 locally or on Deno Deploy. Trace the connection-string handling and base64 decoding involved in the reported error; done means the same MongoDB URI connects successfully on v0.33.0.

Written by the indexing model from the issue text.